Electro-optical device including optical waveguide with slope change point

Abstract

An electro-optical device includes a substrate, an optical waveguide formed on the substrate, a buffer layer formed on the substrate which is provided so as to cover the optical waveguide and an electrode formed on the buffer layer, when viewed in a propagation direction of light, a side surface of the optical waveguide has at least one slope change point. An electro-optical device can reduce the driving voltage and the propagation loss of light.

The invention claimed is: 
     
       1. An electro-optical device comprising:
 a substrate, an optical waveguide formed on the substrate, a buffer layer formed on the substrate and provided so as to cover the optical waveguide, and an electrode formed on the buffer layer, 
 
       wherein:
 the optical waveguide includes:
 a waveguide layer portion including a first side in contact with the substrate and a second side opposite to the first side along a thickness direction, the thickness direction being parallel to a direction along which the substrate, the optical waveguide, the buffer layer, and the electrode are stacked, an entirety of the first side of the waveguide layer portion extending along a plane that is transverse to the thickness direction, and 
 a ridge portion extending from the second side of the waveguide layer portion towards the electrode along the thickness direction, the ridge portion being opposite to the first side of the waveguide layer portion along the thickness direction, the ridge portion including a side surface extending directly from the second side of the waveguide layer portion to an end surface of the ridge portion, the side surface of the ridge portion being transverse to the second side of the waveguide layer portion, and 
 
 when viewed in a propagation direction of light, the side surface of the ridge portion of the optical waveguide includes:
 a plurality of slope change points starting at a position that is 50% or more of a maximum thickness of the optical waveguide starting from the first side of the waveguide layer portion and extending towards the end surface of the ridge portion along the thickness direction, and 
 a plurality of straight lines extending between the slope change points, wherein angles formed by the plurality of straight lines and a first surface of the substrate decrease in a direction extending from the second side of the waveguide layer portion towards the end surface of the ridge portion along the thickness direction. 
 
 
     
     
       2. The electro-optical device according to  claim 1 , wherein
 the side surface of the ridge portion includes a first segment that directly extends from the second side of the waveguide layer portion to a point that is positioned in between the second side of the waveguide layer portion and the end surface of the ridge portion along the thickness direction, and 
 when viewed in the propagation direction of light, an angle formed by the first segment of the side surface of the ridge portion and the first surface of the substrate is 60 degrees or more, the first surface of the substrate being in contact with the first side of the waveguide layer portion. 
 
     
     
       3. The electro-optical device according to  claim 1 , wherein a width of the end surface of the ridge portion of the optical waveguide is in a range of 40% to 75% of a width of a portion of the ridge portion that directly extends from the second side of the waveguide layer portion to a position that is in between the second side of the waveguide layer portion and the end surface of the ridge portion along the thickness direction, the width being transverse to the thickness direction. 
     
     
       4. The electro-optical device according to  claim 1 , wherein
 the side surface of the ridge portion includes a second segment that directly extends from the end surface to a point that is positioned in between the second side of the waveguide layer portion and the end surface of the ridge portion along the thickness direction, and 
 when viewed in the propagation direction of light, an angle formed by the second segment of the side surface of the ridge portion and the first surface of the substrate is larger than 0 degree and is 80 degrees or less, the first surface of the substrate being in contact with the first side of the waveguide layer portion. 
 
     
     
       5. The electro-optical device according to  claim 1 , wherein the optical waveguide is a film composed of LiNbO 3  or LiTaO 3 . 
     
     
       6. The electro-optical device according to  claim 1 , wherein the optical waveguide is a film formed by doping LiNbO 3  with at least one element selected from Ti, Mg, Zn, In, Sc, Er, Tm, Yb and Lu. 
     
     
       7. The electro-optical device according to  claim 1 , wherein the optical waveguide is an epitaxial film. 
     
     
       8. The electro-optical device according to  claim 7 , wherein a crystal orientation of the epitaxial film is along a direction intersecting with the substrate. 
     
     
       9. The electro-optical device according to  claim 1 , wherein
 the side surface of the ridge portion includes:
 a first segment that directly extends from the second side of the waveguide layer portion to a point that is positioned in between the second side of the waveguide layer portion and the end surface of the ridge portion along the thickness direction, and 
 a second segment that directly extends from the end surface of the ridge portion to a point that is positioned in between the second side of the waveguide layer portion and the end surface of the ridge portion along the thickness direction, 
 
 when viewed in the propagation direction of light, a first angle formed by the first segment of the side surface and the first surface of the substrate is 60 degrees or more, and a second angle formed by the second segment of the side surface and the first surface of the substrate is larger than 0 degree and is 80 degrees or less, and the first angle is larger than the second angle, and 
 the first surface of the substrate is in contact with the first side of the waveguide layer portion. 
 
     
     
       10. The electro-optical device according to  claim 9 , wherein the side surface of the ridge portion extends continuously along a plane from the second side of the waveguide layer portion to a first slope change point that is at the position of 50% or more of the maximum thickness of the optical waveguide starting from the first side of the waveguide layer portion and extending to the end surface of the ridge portion along the thickness direction. 
     
     
       11. The electro-optical device according to  claim 1 , wherein the first surface of the substrate is in contact with the first side of the waveguide layer portion, and an entirety of the first surface of the substrate extends along a plane that is transverse to the thickness direction. 
     
     
       12. The electro-optical device according to  claim 11 , wherein the ridge portion is opposite to the first surface of the substrate along the thickness direction. 
     
     
       13. The electro-optical device according to  claim 1 , wherein the side surface of the ridge portion extends continuously along a plane from the second side of the waveguide layer portion to a first slope change point that is at the position of 50% or more of the maximum thickness of the optical waveguide starting from the first side of the waveguide layer portion and extending to the end surface of the ridge portion along the thickness direction.